CAUTION- All evidence hierarchies are an oversimplification. Big data clinical studies/analysis, qualitative research, action research, etc. are often ignored or under-appreciated.  Pyramids like this one are useful visual tools, but be aware there is nuance they cannot capture.

Evidence provided by research is often discussed as a hierarchy. The most robust evidence, systematic reviews, are at the very top of the evidence pyramid, because they are the most intensively reviewed. 

Filtered Information

• Systematic Review: Authors have systematically searched for, appraised, and summarized all of the published research (medical literature) for a specific topic. Sometimes includes a meta-analysis, summarizing the results of multiple studies in a single statistical analysis.

• Critically Appraised Topic: Authors evaluate and synthesize multiple research studies.

• Critically Appraised Articles: Authors evaluate and synopsize individual research studies.

Unfiltered Information

• Randomized Controlled Trial: Clinical trial that includes a randomized group of patients in an experimental group and a control group. These groups are followed up for the variables/outcomes of interest. "Gold standard" is an often used phrase to discuss high quality RCT.
• Cohort Study: Clinical trial that identifies a group (cohorts) of patients, with a shared characteristic, and following the cohort forward for the outcome of interest, may sometimes include a control cohort.

• Case-Control Study/Retrospective Study: Clinical trial that identifies patients who have the characteristic of interest (cases) and control patients without that characteristic, and looks for exposure of interest.

• Background Information/Expert Opinion: Handbooks, encyclopedias, and textbooks often provide a good foundation or introduction and often include generalized information about a condition. While background information presents a convenient summary, often it takes about three years for this type of literature to be published. Expert opinions are often found in scholarly sources as Review Articles (NOTE- these literature reviews or topic reviews are different than Systematic Reviews).

For more info, check out this Quantitative Study Designs guide from Deakins University in Australia. They also offer information about Qualitative Study Design, and a video which is useful to understanding Qualitative vs. Quantitative Study Design. 

Authors of a systematic review;

• ask a specific clinical question,
• perform a comprehensive literature search,
• eliminate the poorly done studies
• and attempt to make practice recommendations based on the well-done studies.

A systematic review with meta-analysis combines all the results of all the studies into a single statistical analysis of results.

Evidence summaries & evidence guidelines

Sometimes you need to search the primary literature. Read below to determine the best evidence you are likely to find based on the domain of the study or research, In PubMed MEDLINE and CINAHL you can use limiters to search for only Clinical Trials.